Method for reducing the consumption of a cryostat and a device for carrying out said method

ABSTRACT

A fluid to be maintained in liquid phase within a cryostat is cooled by means of an auxiliary refrigeration circuit in which an identical fluid is liquefied by the Joule-Thomson expansion process. The auxiliary circuit is supplied with gaseous fluid at a temperature above its Joule-Thomson inversion temperature. The gaseous fluid which is discharged from the cryostat itself also serves to cool the gaseous fluid supplied to the auxiliary circuit and to cause its temperature to drop below the inversion temperature.

[451 July 1,1975

United States Patent Roubeau SHEET FIG. 2

SHEET P7 A P., e, n@ P2 Q I I n I I.\ RTI I M XMI II fr( l I IML I nu \IQ l 4 2\\ P23 FIG. 3

METHOD FOR REDUCING THE CONSUMPTION OF A CRYOSTAT AND A DEVICE FOR CARRYING OUT SAID METHOD This invention relates to a method for reducing the consumption of a cryostat in which it is desired to maintain a fluid in its liquid phase and to a device for carrying out said method. The invention finds an application especially in the construction of a liquid helium economizer.

lt is well known that the isenthalpic expansion of a gas from a medium at the pressure P, to another medium at the pressure P.r P, (across a valve, a porous wall or any other device which is capable of affording resistance to flow) is accompanied by a temperature variation, the magnitude and direction of which are dependent on a coefficient referred-to as the differential coefficient of the Joule-Thomson effect. In the case of a given gas and a given pair of pressures, said coeffcient can be either positive, negative or zero according to the temperature. The temperature above which there no longer exists any pair of pressures P, and P2 corresponding to a negative coefficient is referred-to as the Joule-Thomson effect inversion temperature. At a temperature below this inversion temperature, the isenthalpic expansion may be accompanied by cooling of the gas.

The application of the Joule-Thomson effect below the inversion temperature to the liquefaction of gases isicommon practice. A compressed gas is admitted into a heat exchanger, expands through an impedance and then, as it returns through the heat exchanger, extrac'ts heat from the compressed gas which enters the heat exchanger, with the result that said compressed gas arrives at an even lower temperature than the gas which was previously admitted; after expansion, it again picks up heat from the mass of gas which follows and so on in sequence, with the result that partial or total liquefaction of the gas occurs upon expansion.

Economizers of known types make use of an auxiliary refrigeration circuit in which the gas is liquefied by the Joule-Thomson expansion process. [n systems of this type, however, it is necessary to resort to the use of an auxiliary source of liquid with a view to supplying this latter to a heat exchanger which has the intended function of precooling the gas injected into the auxiliary refrigeration circuit in order to cause the temperature to fall below the inversion temperature. The known economizer systems therefore call for a source of liquid` pipes for transferring said liquid between the source and the heat exchangers of the auxiliary refrigeration circuit, and a large number of siphons. The corresponding installation is therefore cumbersome and, in the final analysis, the economy' which is achieved is of a very low order and rapidly' tends to fall to zero if the power of the installation is not of very high value.

This invention is directed to a method for reducing the consumption ofa cryostat and to a device for carrying out said method in which the disadvantages attached to known systems of this type are eliminated.

ln precise terms, the present invention relates to a method for reducing the consumption of a cryostat in which a fluid is maintained in its liquid phase and cooled in accordance with said method by means of an auxiliary refrigeration circuit in which an identical fluid is liquefied by the method of isenthalpic expansion or so-called Joule-Thomson expansion process, characterized in that said auxiliary' refrigeration circuit is fed with gaseous fluid at a temperature which is higher than the Joule-Thomson effect inversion temperature in the case of said fluid and that the gaseous fluid which passes out of the cryostat itself serves to perform a contributory function in cooling said gaseous fluid supplied to the auxiliary refrigeration circuit and to cause the temperature of said gaseous fluid to drop below said inversion temperature,

The invention further relates to a device for reducing the consumption of fluid of a cryostat which serves to carry out the method hereinabove defined and comprises a refrigeration circuit of the type employed for liquefaction by Joule-Thomson expansion.

ln a first alternative embodiment, the economizer is characterized in that it comprises:

a source of gas at a pressure P1 and at a temperature which is higher than the Joule-Thomson effect inversion temperature of said gas,

a heat exchanger comprising a high-pressure circuit supplied with gas from said source and a lowpressure circuit supplied both with the nonliquefied portion of the gas after expansion and with the vaporized portion of the liquid contained in the cryostat,

a gas expansion unit located at the outlet of said highpressure circuit. the pressure P2 of the expanded gas at the outlet of said unit being the pressure which prevails within the cryostat. the liquefied portion of said expanded gas being returned into the liquid contained in said cryostat.

the overall size of the assembly consisting of heat exchanger and expansion unit being sufficiently small to permit introduction of said assembly through the neck ofsaid cryostat which is sealed by means ofa leak-tight plug throuch which are passed the two circuits for the circulation of gases at the pressures P, and P.

ln a second alternative embodiment, the economizer is characterized in that it comprises:

a pump-compressor assembly' for sucking the gas at a pressure P'. I which is lower than atmospheric pressure and discharging said gas at a pressure P', which is higher than P'.

a heat-exchanger and expansion-unit assembly comprising successively in the direction of circulation of the fluid:

a. a high-pressure circuit supplied with gas at the pressure P', by said compressor, in which the gas undergoes a pressure drop AP b. a gas expansion unit such that the gas on the upstream side of said unit is at the pressure P, P', AP, and the gas on the downstream side of said unit is at the pressure P2 P c. a vaporization heat-exchanger in which the portion of the gas which has liquefied after passing through the expansion unit is vaporized at the pressure P2 while cooling the liquid ofthe cryostat and/or while condensing the vapors of the liquid of the cryostat,

d. a condensation heat-exchanger through which circulates the gas discharged from the vaporization heat-exchanger, said gas being heated while condensing the vapors of the liquid of the cryo- Slal,

e. a low-pressure circuit through which passes the gas discharged from said condensation heatexchanger, said low-pressure circuit being in close thermal contact with said high-pressure circuit so as to constitute a main heat exchanger` said gas being subjected to a pressure drop AP- i within said low-pressure.` circuit which delivers a gas to the pump-compressor assemblyl at thc pressure P-, P2 APL` a circuit for the flow of the vaporized portion of the liquid contained in said cryostat and at the pressure P;i in close thermal coupling with said high-pressure and Iowpressure circuits. said vaporized portion being subjected within said circuit to a pressure drop AP so as to be discharged at the pressure P3 API, which is equal in particular to atmospheric pressure` the overall size of said heat-exchanger and expansionunit assembly being sufficiently' small to permit of in troduction of said assembly into the neck of said cryostat and this latter being closed by means of a leak-tight plug through which are passed the three circuits for the circulation of gases at the pressures P, AP,. P2 .AP-z` Ps Apri- The expansion units employed can be either a capillary tube, an expansion valve. a porous medium. a throttle valve` or any other means of creating a backpressure by producing expansion as a result of a throttling or wiredrawing effect.

These two alternative embodiments permit a number of different modes of execution` depending on the values of the high pressure P,` of the low pressure P.1 and of the pressure P3 ofthe main bath of the cryostat.

By way of example and in the case of a liquid helium economizer` it is possible to have.

a. in the first alternative embodiment: P, 3 to 20 atmospheres P2 atmospheric pressure -l- AP2 Pri P2 AP, AP2

= 0.01 to 0.l atmosphere.

b. in the second alternative embodiment:

P, 3 to 20 atmospheres P2 0.05 to 0.2 atmosphere P1, atmospheric pressure -l- APf,

But it is also possible to have:

P, 3 to 20 atmospheres PE 0.02 to 0.05 atmosphere P,l P.l (first alternative embodiment) which would be the case in particular of a cryostat employed below the temperature ofthe point of helium (Tt=2l 7 K and Pt=0.05 atmosphere). or which would be the case ofa cryostat in which the bath of ordinary helium serves to condense the isotope-3 of helium ofa separate circuit for cooling at a lower temper ature (a circuit for pure helium-3 or a circuit for heli um-3 of a dilution refrigerator).

In accordance with another mode ofexecution of the first alternative embodiment. a part of the gas can be directed towards a compressor or a pumpcompressor assembly constituting the source of gas at the pressure P,. Similarly` in a particular mode of execution of the second alternative embodiment, Athe pump can be inde pendent ab@ the gas at the pssure P, can he supplied from a soue other than compressor.

The charateristic featufs and advantages of the invention will IB any case bome more readily' apparent from the follW'ing description of examples ofbnstruction which a given by Way of explanation 'tl not in any sense by way of limitation` reference being made to the accompanying drawings, wherein:

FIG. l is a diagram of the device of the invention in accordance with the first alternative embodiment;

FIG. 2 is a diagram of thc device of the invention in accordance with the second alternative embodiment',

FIG. 3 is a simplified diagram of a heat exchanger which can be employed in the first alternative ernbodiment;

FIG. 4 shows a particular form of construction of the economizer in accordance with the second alternative embodiment ofthe invention;

FIG. 5 shows two details A and B of the economizer of FIGY 4;

FIG. 6 shows two further details C and D ofthe same economizer.

Although the invention can be utilized for a wide range of different gases` the following description rcfers by way of explanation to a cryostat which contains liquid helium. The inversion temperature of the Joule- Thomson effect in the case of helium is in the vicinity of 50K. Above this temperature. the helium therefore becomes heated upon expansion. However, this temperature rise is ofa very low order since it corresponds to an extremely small difference in specific heat between the gas under pressure and the expanded gas. For example, in the case of helium. the difference in enthalpy' at 300K between the gas at IO atmospheres and the gas at OI atmosphere is approximately 3 J/g. lf the helium injected into the economizer is at a temperature higher than 50K, for example at ambient temperature` it is therefore apparent that only a few frigories between 50K and ambient temperature will be necessary in order to Iiquefy said gas by the cumulative process of `lciuleThomson expansion.

In accordance with the invention, the frigories aforesaid are taken from the helium gas which passes out of the c'ryostat and evaporates at 4K, for example. The difference in enthalpy of the gas between 4K and 300K is approximately l600 J/g. It is therefore necessary to have only a very low gas flow rate in order to cool the gas introduced into the economizer to a suffcient extent to ensure that the Joule-Thomson expansion can produce a cooling action.

In FIG. l, the cryostat 2 of the Dewar vessel type contains liquid helium 4. A refrigeration circuit 8 is introduced into the neck 6 of said cryostat` said circuit being supplied from a source l0 of helium under pressure and at a temperature above the Joule-Thomson effect inversion temperature of helium. The heat exchanger !2 comprises a high-pressure circuit 14 and a low-pressure circuit 16. An expansion unit I8 is located at the extremity of the high-pressure circuit 14. On the downstream side of said expansion unit, the liquefied portion of the expanded helium is returned to the helium contained within the cryostat. The low-pressure circuit I6 is fed with gas both from the non-liquefied portion of the gas after expansion of this latter through the expansion unit I8 and from the vaporized portion of the liquid 4 contained in the cryostat. A plug 20 serves to close the vessel at the top of the neck.

If AP, and AP'2 designate the pressure drops within the high-pressure circuit 14 and low-pressure circuit I6, respectively` the source I0 delivers helium at the pressure P, AP which is again present on the upstream side of the expansion unit at the pressure P,. On the downstream side of said unit the pressure above the liquid 4 is P2 and, at the outlet ofthe low-pressure circuit 16, the helium escapes at the pressure P2 AF2 which can be atmospheric pressure.

In FIG. 2` there is shown the device ofthe invention in accordance with the second alternative embodiment. A pump-compressor assembly draws-up the gas at a pressure P2 APg and discharges the gas at a pressure PI AP,.

The helium then flows successively through the fol lowing units:

a. a high-pressure circuit 52 in which a pressure drop AP, occurs;

b. an expansion unit 54 in which the helium pressure is P, on the upstream side of said unit and P2 on the downstream side;

c. a vaporization heat-exchanger 56 in which the helium S8 which has been liquefied at the outlet of the expansion unit 54 is vaporized at the pressure P2 and cools the liquid 60 of the cryostat 62 or. if necessary and depending on the level of the liquid within the cryostat 62, condenses the vapors 64 of the evaporated liquid 60'.

d. a condensation heat-exchanger 66 in which that portion of the helium 58 which has vaporized within the heat exchanger 56 is heated from the temperature corresponding to the saturated vapor pressure P2 up to the ideal condensing vapor temperature corresponding to the pressure P.1 which prevails within the cryostat (and which can be in the vicinity of atmospheric pressure, for example), the temperature rise of said gas 30 being caused by condensation of part of the vapors 64 A of the main bath 60 which return to this latter in liquid form;

e. a low-pressure circuit 68 which forms in particular with the high-pressure circuit S2 a heat exchanger 70, 35

the pressure drop within said circuit 68 being AF2.

The vaporized portion 64 ofthe evaporated liquid 60 is discharged through a circuit 72 in which there is al pressure drop APH, with the result that the pressure of said vapor is caused to change from P3 to P3 AP the circuit 72 also forms part` of the h'eat exchanger '70. The neck ofthe cryostat is closed by means of a plug 74.

Experiments performed by the present Applicant have demonstrated that the helium losses of a cryostat are reduced by more than one order of magnitude when said cryostat is equipped with the device according to the invention, The losses thus reduced are those which are caused by supports placed in a vacuum. by radiation. by the Joule effect in situ. by eddy currents. One advantageous form of construction of the heat exchanger 12 is illustrated in FIG. 3. In this figure, the heat exchanger is constituted by a hollow cylinder 30 and especially a metallic cylinder surrounding a plurality of tubes 32 which are fed in parallel with expanded gas at the pressure P2 (arrows 34); these hollow tubes constitute the secondary circuit of the heat exchanger. The primary circuit is constituted by the space formed between the tubes 32 and the cylinder 30. This circuit is associated with an inlet 36 for the admission of4 gas at the pressure Pl -l- API and with an outlet 38 through which the gas is discharged at the pressure Pl and conveyed to the expansion unit 18,

Since the helium flows in a laminar regime within the tubes 32, it is known that the heat exchange process is not dependent on the diameter of the tubes but on their length (WA H. McAdams, Transmission ofheat, Chapt. 9.2); these tubes must accordingly have the smallest diameter which is compatible with an acceptable pressure drop.

A calculation which is simplified but has been con lirmed by practical experience shows that a tube 30 centimeters in length is capable of exchanging the enthalpy corresponding to 2 or 3 milliwatts of refrigeration whilst the corresponding flow rate of helium is approximately l04 g/s.

Since the pressure drop is proportional to the fourth power ofthe reciprocal of the diameter it is shown by calculation, for example, that the diameter must be larger than 0.5 mm in order to have a pressure drop AP below 0.1 atmosphere with P2 l atmosphere and must be larger than 0.6 mm in order that AP should be below 0.08 atmosphere with P2 0.1 atmosphere.

With a diameter of 0.6 mm, P2 0.1 atmosphere, a length of 35 centimeters and a flow rate of 10* g/s per tube, the present Applicant has observed experimentally a pressure drop AP of 0.0025 atmosphere. Taking into account the above-mentioned law. the diameters usually employed will therefore be within the range of 0.5 to 0.7 mm.

So far as the high-pressure circuit is concerned, the problem of pressure drop does not arise and the tubes must be as closely spaced as possible while making allowances for the machining of the centering rings.

There is shown in FlGS. 4 to 6 one particular form of construction of an cconomizer in accordance with the second alternative embodiment of the invention.

FIG. 4 shows the cconomizer unit as a whole. The helium is introduced through the high-pressure inlet which communicates with an admission tube 102. The helium passes successively through the main heat exchanger 104, the tube 140, a porous medium 106 which performs the function of expansion unit, the vaporization heat-exchanger |08, the condensation heatexchanger 110, the tube or closed, hollow shell 112, and is finally discharged through the outlet connection 114. The outlet 116 guides the gas as this latter passes 0 out of the main bath ofthe cryostat (not shown) which is assumed to surround the vaporization and condensation heat-exchangers 108 and 110; said gas is collected in the annular space formed between the tube 112 and the outer tube or shell 118 which are located within the bead of the cryostat.

FIG. 5 shows two details of the device of FIG. 4 as considered at the levels A and B. ln FlG. SA, the highpressure gas is admitted through the tube 102 at the pressure P, -l- API. At the extremity 120, said highpressure gas spreads out and circulates within the shell 160 between the tubes 122 through which the lowpressure helium circulates in the opposite direction and is discharged at the extremities 124 at the pressure P2 AF2. The helium which leaves the main bath of the cryostat flows through a small number of pressure-drop tubes AP3, only one tube 126 being shown in FIG. 5. Said helium is discharged at the pressure P:L AP into the cham ber formed by the two rings or tube sheets 130 and 132 and escapes through the orifice 134 into the annular space formed between the cylinders 112 and 1 18.

ln FIG. 5B, the high-pressure gas which circulates between the tubes 122 is returned through the tube 140 at the level ofthe ring or tube sheet 142. The helium which is evaporated from the main bath at the pressure P3 penetrates into the circuit through the orifice 144, then into the tubes 126. Between the rings 130 and 142, provision is accordingly made for a heat ex` changer having three circuits through which gases are circulated at the pressures P li2 and P3.

ln FIG` 5` the references 143 and 145 designate respectively one of the spacer rings or tube sheets and a central rod which is intended to prevent any preferential llow along the axis` FIG. 6 illustrates two details at the levels C and D. ln FlG` 6C, the high-pressure gas is admitted through the tube 140. The expanded gas at the pressure P2 is collected at the extremities 150 of the tubes 122 in the proximity of thc ring or tube sheet 152. The so-called condensation heat-exchanger is therefore provided between the rings 142 and 152. The condensed liquid escapes through the orifice 154 and is returned to the main bath.

The expansion unit 106 of FIG. 6D is fed with helium at the pressure Pl through the tube 140. Expansion takes place through three washers 158 ofsintered stainless steel. The liquefied portion of the helium is collected at the lower end of the shell 160 which constitutes the so-called vaporization heat-exchanger 108.

ln this form of construction, the tubes 122, 126, 140 and 160 are advantageously of stainless steel, the thickness of the tube walls being between O.l and 0.2 mm. The rings or tube sheets 130, 132, 142 and 152 are of copper and are brazed to the stainless steel tubes.

ln the form of construction shown in FIGS. 4 to 6, the tubes 122 which constitute the low-pressure circuit have the same cross-section from one end of the heatexchanger to the other. It is possible in an improved am rangement to make use of smaller tubes in the lower portion ofthe heat exchanger in which the temperature is at its lowest value since the pressure drop varies approximately as the square of the temperature. ln this case` the tubes 122 can have a minimum diameter which is two to three times smaller within said lower portion.

From the foregoing description. it is apparent that the economizer in accordance with the invention has a large number of advantages over known devices of this type:

l. it avoids the use of an auxiliary source of liquid helium for the purpose of reducing the temperature ofthe gas to a value below the inversion temperature. thus simplifying the economizer and increasing its efficiency,

2. it utilizes the property of unbalanced mass flows throughout the heat exchanger and accordingly secures the advantages:

a. of an increasing temperature difference from the cold end to the hot end and this temperature difference in conjunction with the increase in heat conductivity within the highest-temperature zone permits the performance ofthe greater part ofthe heat exchange process with a small surface area` b. of low pressure drops as a result of the low viscosity of the gas within the cold portion which is the largest (9() 7r of the surface area below 77K),

3. it does not cause any temperature jump in the gas during cooling since,the stream which undergoes a temperature rise is always at the highest possible temperature permitted by the thermodynamic balance, which produces the minimum entropy and consequently permits the maximum efficiency` 4. it makes use of an improved heat exchanger of simple construction which makes it possible:

a. to reduce the pressure losses on the low-pressure side to a minimum value under conditions of equal heat exchange and consequently lo choose a low value for P. .1 with the result that the cooling effect S can be increased to a very appreciable extent;

b. to ensure maximum heat exchange with the minimum longitudinal conductivity in the metal of the heat exchanger,

c to have a very high degree of tolerance to impurilU ties by virtue ofthe large volume left for the gas on the high-pressure side,

d. to prevent "channeling" by virtue of the fact that the greater part of the pressure drops at the hot end s which is the most effectively thermalized is localb. in the event that the pressure P2 is of the order of O.l atmosphere and that a triple-flow heatexchanger is employed` in which ease the maximum efficiency is achieved.

What l claim is:

1. An auxiliary refrigeration unit for reducing the consumption ofa fluid existing in liquid and vapor phases within a cryostat, the unit adapted to be connected to a source of high pressure fluid for supply at a temperature which is higher than the Joule-Thomson effect inversion temperature of the fluid, comprising:

a hollow cylinder adapted to be disposed in the cryostat and having an inlet connected to the source of pressurized gas to establish a high pressure flow circuit for the pressurized gas, a gas expansion unit connected to the outlet of said cylinder for expanding said pressurized gas in accordance with the Joule-Thomson effect. the pressure of the expanded gas at the outlet of said unit being at the pressure which prevails within the cryostat, and the liquefied portion of said expanded gas being returned into the liquid contained in said cryostat, and

a plurality of tubes extending through said cylinder and adapted to receive both the nonliquefled portion of said gas after expansion and the vaporized portion of the liquid contained in the cryostat to establish a low pressure flow circuit for said portions arid pass said portions in a heat exchange relation to said pressurized gas passing through said cylinder.

2. An auxiliary' refrigeration unit for reducing the consumption ofa fluid existing in liquid and vapor phases within a cryostat. including a heat exchanger and expansion unit assembly' adapted to be connected to a pump-compressor assembly of the type having a gas inlet and a gas outlet for supplying high pressure gas at a temperature above its Joule-Thomson effect inversion temperature. said h'eat exchanger and expansion unit assembly comprising:

a. a cylindrical shell adapted to be partially immersed in the cryostat liquid;

b, a plurality of tube sheets, said tube sheets dividing said cylindrical shell into a plurality of chambers including an inlet chamber having means for connection to the outlet of the pump compressor assembly; an outlet chamber having means for connection to the inlet of the pump-compressor assembly'` a main heat exchanger chamber; a condensation heat exchanger chamber; and a vaporization heat exchanger chamber; said cylindrical shell having formed therein a plurality of apertures for permitting entry of a portion ofthe vapor from within the cryostat into said condensation heat exchanger chamber; a plurality oflongitudinal parallel low pressure circuit tubes connecting said outlet chamber with said vaporization heat exchanger chamber, said low pressure circuit tubes extending through said main heat exchanger chamber and said condensation heat exchanger chamber to divide the space within said main heat exchanger chamber and said Condensation heat exchanger chamber into shell sides and tube sides;

d. an admission tube connecting said inlet chamber with said main heat exchanger chamber shell side, and a main heat exchanger outlet tube connecting said main heat exchanger chamber shell side with said vaporization heat exchanger chamber and extending through said condensation heat exchanger Chamber;

e. a gas expansion unit connected to said main heat exchanger outlet tube and being disposed within said vaporization heat exchanger chamber.

3. The auxiliary refrigeration unit defined by claim 2 wherein said heat exchanger and expansion unit assembly further comprises:

a. an additional pair of longitudinally spaced tube sheets disposed adjacent the inlet end of said main heat exchanger chamber and within said cylindrical shell to define a vapor circuit chamber;

b. a jacket mounted on said cylindrical shell for defining a vapor collection space around said cylindrical shell and adjacent said vapor circuit chamber, said jacket having vapor inlet means Communicating with the shell side of said vapor circuit chamber and vapor discharge means; and

c. a vapor circuit tube connecting said condensation heat exchanger chamber with said vapor circuit chamber and extending through said main heat ex changer chamber.

4. The auxiliary refrigeration unit defined by claim 2 further including means mounted within said main heat exchanger chamber for preventing preferrential flow of the high pressure fluid through said shell side of said main heat exchanger chamber.

5. The auxiliary refrigeration unit defined by claim 2 wherein said jacket which defines said vapor collection space comprises a cylindrical housing having a diamc- 5 ter greater than the diameter of said cylindrical shell, said cylindrical housing being mounted concentric with said cylindrical shell and between said vapor circuit chamber and said outlet chamber.

6. A device for reducing the consumption of a cryostat containing a fluid in a liquid phase. comprising:

a hollow cylinder disposed in said Cryostat and having an inlet for receiving said fluid in the form of a gas under pressure and at a temperature which is higher than the Joule-Thomson effect inversion temperature of said gas,

a gas expansion unit connected to the outlet of said cylinder for expanding said pressurized gas in ac cordance with the Joule-Thomson effect. the pressure of the expanded gas at the outlet of said unit being at the pressure which prevails within the cryostat.

a vaporization heat-exchanger disposed in said cryostat and connected to said gas expansion unit for vaporizing that portion of the gas which has liquefied after passing through the expansion unit while cooling the liquid in the cryostat and/or while condensing the vapors of the liquid of said cryostat,

a condensation heat-exchanger disposed in said cryostat and connected to said vaporization heat exchanger for receiving the gas discharged from the vaporization heat-exchanger and the vapors of the liquid of the cryostat. and passing said gas and said vapors in a heat exchange relation to heat said gas and condense said vapors,

a pump-compressor assembly for drawing the heated gas from said condensation heat exchanger and pressurizing said gas. the outlet of said assembly being connected to said inlet of said cylinder, and

a first and second series of tubes extending through said cylinder and adapted to receive the gas from said gas expansion unit and the vaporized portion of the liquid contained in the cryostat` respectively. said tubes being adapted to pass said gas and said vaporized portion in a heat exchange relation to said pressurized gas passing through said cylinder.

7. The device of claim 6 wherein a flow circuit connects said condensation heat exchanger to said pump compressor assembly and passes said heated gas in a 50 heat exchange relationship to the pressurized gas passing through said cylinder and to said gas and said vaporized portion passing through said tubes, 

1. An auxiliary refrigeration unit for reducing the consumption of a fluid existing in liquid and vapor phases within a cryostat, the unit adapted to be connected to a source of high pressure fluid for supply at a temperature which is higher than the JouleThomson effect inversion temperature of the fluid, comprising: a hollow cylinder adapted to be disposed in the cryostat and having an inlet connected to the source of pressurized gas to establish a high pressure flow circuit for the pressurized gas, a gas expansion unit connected to the outlet of said cylinder for expanding said pressurized gas in accordance with the Joule-Thomson effect, the pressure of the expanded gas at the outlet of said unit being at the pressure which prevails within the cryostat, and the liquefied portion of said expanded gas being returned into the liquid contained in said cryostat, and a plurality of tubes extending through sAid cylinder and adapted to receive both the nonliquefied portion of said gas after expansion and the vaporized portion of the liquid contained in the cryostat to establish a low pressure flow circuit for said portions and pass said portions in a heat exchange relation to said pressurized gas passing through said cylinder.
 2. An auxiliary refrigeration unit for reducing the consumption of a fluid existing in liquid and vapor phases within a cryostat, including a heat exchanger and expansion unit assembly adapted to be connected to a pump-compressor assembly of the type having a gas inlet and a gas outlet for supplying high pressure gas at a temperature above its Joule-Thomson effect inversion temperature, said heat exchanger and expansion unit assembly comprising: a. a cylindrical shell adapted to be partially immersed in the cryostat liquid; b. a plurality of tube sheets, said tube sheets dividing said cylindrical shell into a plurality of chambers including an inlet chamber having means for connection to the outlet of the pump compressor assembly; an outlet chamber having means for connection to the inlet of the pump-compressor assembly; a main heat exchanger chamber; a condensation heat exchanger chamber; and a vaporization heat exchanger chamber; said cylindrical shell having formed therein a plurality of apertures for permitting entry of a portion of the vapor from within the cryostat into said condensation heat exchanger chamber; c. a plurality of longitudinal parallel low pressure circuit tubes connecting said outlet chamber with said vaporization heat exchanger chamber, said low pressure circuit tubes extending through said main heat exchanger chamber and said condensation heat exchanger chamber to divide the space within said main heat exchanger chamber and said condensation heat exchanger chamber into shell sides and tube sides; d. an admission tube connecting said inlet chamber with said main heat exchanger chamber shell side, and a main heat exchanger outlet tube connecting said main heat exchanger chamber shell side with said vaporization heat exchanger chamber and extending through said condensation heat exchanger chamber; e. a gas expansion unit connected to said main heat exchanger outlet tube and being disposed within said vaporization heat exchanger chamber.
 3. The auxiliary refrigeration unit defined by claim 2 wherein said heat exchanger and expansion unit assembly further comprises: a. an additional pair of longitudinally spaced tube sheets disposed adjacent the inlet end of said main heat exchanger chamber and within said cylindrical shell to define a vapor circuit chamber; b. a jacket mounted on said cylindrical shell for defining a vapor collection space around said cylindrical shell and adjacent said vapor circuit chamber, said jacket having vapor inlet means communicating with the shell side of said vapor circuit chamber and vapor discharge means; and c. a vapor circuit tube connecting said condensation heat exchanger chamber with said vapor circuit chamber and extending through said main heat exchanger chamber.
 4. The auxiliary refrigeration unit defined by claim 2 further including means mounted within said main heat exchanger chamber for preventing preferrential flow of the high pressure fluid through said shell side of said main heat exchanger chamber.
 5. The auxiliary refrigeration unit defined by claim 2 wherein said jacket which defines said vapor collection space comprises a cylindrical housing having a diameter greater than the diameter of said cylindrical shell, said cylindrical housing being mounted concentric with said cylindrical shell and between said vapor circuit chamber and said outlet chamber.
 6. A device for reducing the consumption of a cryostat containing a fluid in a liquid phase, comprising: a hollow cylinder disposed in said cryostat and having an inlet for receiving said fluid in the form of a gas under pressure and at a temperature which is higher than the JoUle-Thomson effect inversion temperature of said gas, a gas expansion unit connected to the outlet of said cylinder for expanding said pressurized gas in accordance with the Joule-Thomson effect, the pressure of the expanded gas at the outlet of said unit being at the pressure which prevails within the cryostat, a vaporization heat-exchanger disposed in said cryostat and connected to said gas expansion unit for vaporizing that portion of the gas which has liquefied after passing through the expansion unit while cooling the liquid in the cryostat and/or while condensing the vapors of the liquid of said cryostat, a condensation heat-exchanger disposed in said cryostat and connected to said vaporization heat exchanger for receiving the gas discharged from the vaporization heat-exchanger and the vapors of the liquid of the cryostat, and passing said gas and said vapors in a heat exchange relation to heat said gas and condense said vapors, a pump-compressor assembly for drawing the heated gas from said condensation heat exchanger and pressurizing said gas, the outlet of said assembly being connected to said inlet of said cylinder, and a first and second series of tubes extending through said cylinder and adapted to receive the gas from said gas expansion unit and the vaporized portion of the liquid contained in the cryostat, respectively, said tubes being adapted to pass said gas and said vaporized portion in a heat exchange relation to said pressurized gas passing through said cylinder.
 7. The device of claim 6 wherein a flow circuit connects said condensation heat exchanger to said pump compressor assembly and passes said heated gas in a heat exchange relationship to the pressurized gas passing through said cylinder and to said gas and said vaporized portion passing through said tubes. 